@gertvdijk @jpmens for accelerating the specific case of cloning the Linux kernel repo there is an even better option: downloading a cacheable, CDN-distributable, regularly updated bundle file: https://www.kernel.org/best-way-to-do-linux-clones-for-your-ci.html
(Mercurial had this feature available transparently since 2015: https://gregoryszorc.com/blog/2015/05/29/faster-cloning-from-hg.mozilla.org-with-server-provided-bundles/ . Git is playing catchup in this particular aspect since the feature isn't yet in core or, as far as I can tell, on the roadmap.)